1. Field of the Invention
The invention relates to a vehicle drive unit provided with a combustion engine (as will be referred to as the "engine") and an electric-motor/generator (as will also be referred to as the "motor generator") and, more particularly, to a control system for performing a control to maintain the drive unit in a state in which the vehicle can be quickly restarted at a vehicle stopping time.
2. Description of the Related Art
As one mode of the vehicular drive unit, there is a drive unit in which an engine, a motor generator and a transmission are combined, as disclosed in U.S. Pat. No. 4,533,011. By using the motor generator as a power generator, this drive unit is constructed to recover the braking energy from wheels and store it as an electric power, so that the electric power is used for driving the motor generator to start the engine and drive the vehicle. In this unit, moreover, when the vehicle is in the stopping state, the supply of fuel to the engine is stopped to reduce the fuel consumption rate and accordingly the exhaust gas emission.
Incidentally, the drive unit stops the fuel supply to the engine when the vehicle stops to stop the rotation of the engine so that the accessories such as the air conditioner or the alternator connected to and driven by the engine cannot be operated. When the vehicle is to be restarted from the stopping state, the engine putters because it takes a considerable time for the engine to reach a predetermined idling rotation or a rotation matching the throttle opening after the engine is started.
In order to solve this problem, the prior art conceives a control method by which the fuel supply to the engine is exclusively cut while rotating the engine, even at a stop, at an RPM corresponding to the idling RPM of the motor generator (as will be herein called the "motoring"). If this control method is adopted, however, a pumping action of the engine is caused by the motoring so that the load upon the motor generator grows high during the stop. If the stopping time becomes long, the energy, as recovered by the regeneration, is greatly consumed so that a sufficient assist cannot be achieved for the starting or accelerating time. This, in turn, requires much fuel, thus dropping the power performance or deteriorating the mileage improving effect. Furthermore, if a short stop is frequently repeated, the fuel consumption at each restart of the engine deteriorates the mileage improving effect.
Moreover, the vehicle is usually equipped with a catalyst which is arranged in the exhaust system so as to purify the exhaust gas. The reaction promoting function of the catalyst drops if the catalyst is in a low temperature state. If, therefore, the aforementioned pumping of the engine continues for a long time, the catalyst temperature drops so that insufficiently purified exhaust gas is discharged at the engine restarting time.